Recently, as the size and weight of an electrophotographic copying machine and the like have been reduced, the size and weight of a roll for a fixing unit used in its fixing device have been reduced. For example, a roll for a fixing unit is conventionally known in which a sponge layer 2 is formed on a circumferential surface of a metal core 1 and a surface coating layer 3 is formed on an outer circumferential surface of the sponge layer 2, as shown in FIGS. 11 and 12. Since this roll uses the sponge layer 2, the hardness of the roll surface is low. Hence, even when the diameter of the roll is reduced, a sufficient nip width required for thermal fixing can be obtained, and the roll for a fixing unit has advantages such as light weight.
Such a roll for a fixing unit is manufactured in, e.g., the following manner.
1 First, an adhesive is coated on an outer circumferential surface of the metal core 1 and a rubber material is applied on it. The rubber material is heated to be vulcanized and foamed, thus forming the sponge layer 2. Alternatively, using an extruder, the rubber material is formed by continuous extrusion in advance, and the extruded rubber material is heated to be vulcanized and foamed, thereby forming a sponge tube. Then, a metal core is mounted inside the sponge tube to form the sponge layer.
2 Then, a liquid rubber is casted onto an outer circumferential surface of the sponge layer 2 and hardened to form the surface coating layer 3, thereby obtaining a roll. The surface coating layer 3 can be formed by fitting a preformed tube.
Generally, in a fixing unit of an electrophotographic copying machine and the like, heat of a temperature of 150.degree. to 200.degree. C. is applied to the roll for a fixing unit in order to perform thermal fixing. However, in the conventional roll for a fixing unit described above, since the material constituting the sponge layer and the gas in cells thermally expand, a change in outer diameter is large as compared with a polymer material usually used in a roll for a fixing unit, and a change in nip width is also large.
When the temperature of the surface of the roll is not uniform, a local expansion difference occurs in the roll outer diameter, resulting in a non-uniform nip width. For example, when the size of the recording paper is changed and paper continuously passes not over the entire surface but over a partial surface of the roll, only the partial portion where the paper passes is deprived of heat. The nip width is changed due to a difference in thermal expansion between the paper non-contact portion and the paper contact portion, resulting in wrinkles.
When heat is applied to the sponge layer, the pressure inside the cells is increased as the gas thermally expands. Therefore, the cells expand to increase the outer diameter of the roll. At the same time, however, the gas in the cells has a nature to permeate outside the roll through the cell membranes to equilibrate the pressure. As a result, the increased outer diameter of the roll gradually becomes small. In this case, regarding the diffusing state of the gas in the cells, the gas in the cells at the two end portions of the roll can be easily diffused to the two end faces, whereas the gas in the cells at the central portion of the roll cannot easily permeate outside the roll because of the influence of the surface coating layer through which the gas cannot easily permeate. As a result, the change in roll diameter is not uniform between the central roll portion and the two end portions. When a non-fixed image is to be fixed, wrinkles are formed.
Fluoroplastic is often used as a material of the surface coating layer. Although fluoroplastic has a good heat resistance, it can be charged very easily. Thus, the surface of the roll for a fixing unit is charged to several thousands to several ten thousands of volts when it contacts copy paper and the like during thermal fixing. As a result, paper dust and toner are electrically adsorbed to the surface of the roll to impair the high separation performance of the surface of fluoroplastic, resulting in occurrence of toner offset after use for a short period of time.